It is known that these wheels have cutting teeth which, when rubbed on a flint, produce a spark for lighting. These teeth must be made with sufficient precision and regularity so that the operation of the wheel is satisfactory. Presently-known processes for the making of such teeth are: milling by generation, milling tooth by tooth, rolling by the "Rotor-Flow" process and chisel striking.
Milling by generation and milling tooth by tooth require the use of costly machines and tools (special cutters). In addition, the life of these tools is short especially when frequent sharpening is called for, and sharpening is necessarily frequent in this case because, to ensure proper quality, these lighter wheels must be made of hard alloy steels.
Milling by the "Roto-Flow" process does not always make it possible to obtain the desired geometrical form of the teeth, especially when the toothing has a re-entrant angle.
Chisel striking is the most suitable method provided that use is made of a machine fulfilling certain imperative requirements: rigidity, reliability and accuracy of its mechanism, which alone make it possible to ensure the rapid execution of the teeth, the desired precision of the teeth, the exactitude of tooth form, the uniformity and regularity of toothing in mass production runs, the possibility of varying the diameter and the number of teeth on the wheel, the facility of the required adjustments, the possibility of using standard, commercially-available chisels for striking, the speed with which these chisels are refitted after sharpening, automatic operation of the machine or operation with a stop after each cycle.
Existing machines do not always fulfil these conditions. In those which employ chisel striking, this striking is obtained by flying impact and then rebounding of the chisel which then works only under inertia, in the manner of a hammer. If the required mechanism is simplified, in particular owing to the fact that it is then not necessary to slow down the workpiece during the striking which takes place "on the fly," the accuracy of the impact and, consequently, the regularity of the teeth obtained remain uncertain.